temperature, osmotic regulation and the urinary system chapter 49

Temperature, Osmotic Regulation and the Urinary System

Chapter 49

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Regulating Body Temperature

The rate of any chemical reaction is affected by temperature

-The Q10 is the ratio of reaction rates at two temperatures that differ by 10oC

-For most enzymes, Q10 is around 2

Most organisms have a Q10 for metabolic rate around 2 or 3

-Thus, the effect of temperature is mainly on the enzymes involved in metabolism

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Body temperature is determined by internal factors, such as metabolism, external factors that affect heat transfer, as well as behavior

Body heat = heat produced + heat transferred

-Note that the heat transferred can be either positive or negative

-Can be used for both heating and cooling

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Four mechanisms of heat transfer are relevant to biological systems

-Radiation = By electromagnetic radiation

-Conduction = Directly between two objects

-Convection = By the movement of a gas or liquid

-Evaporation = Conversion of water to gas

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Heat transfer also depends on other factors, that influence these four physical processes

-Surface area to mass ratio

-Difference between ambient and body temperature

-Specific heat conduction

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Classification of Organisms

For many years, animals were classified according to whether they maintained a constant body temperature

-Homeotherms = Regulate their body temperature about a set point

-Also called “warm-blooded”

-Poikilotherms = Allow their body temperature to conform to the environment

-Also called “cold-blooded”

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Classification of Organisms

Limitations to this dichotomy led to another view based on how body heat is generated

-Endotherms = Use metabolism to generate body heat and maintain temperature above ambient temperature

-Ectotherms = Do not use metabolism to produce heat and have body temperature that conforms to ambient temperature

Heterotherms fall between these extremes

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Ectotherms

Ectotherms regulate temperature using behavior

-Insects, such as moths, use a shivering reflex to warm thoracic muscles for flight

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Ectotherms

Many marine animals, such as killer whales, limit heat loss in cold water using countercurrent heat exchange

-Warm blood pumped from within the body in arteries warms the cooler blood returning from the skin within veins

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Ectotherms

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Ectotherms

Reptiles place themselves in varying locations of sunlight and shade

-Some can maximize the effect of behavioral regulation by also controlling blood flow

In general, ectotherms have low metabolic rates, which have the advantage of low energy intake

-However, they are not capable of sustained high-energy activity

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Endotherms

A high metabolic rate can be used to warm the endotherm if it is cold

The simplest way to regulate body temperature is by the control of blood flow to the surface of the animal

-Vasodilation increases blood flow, thereby increasing heat dissipation

-Vasoconstriction decreases blood flow, thus limiting heat loss

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Endotherms

When ambient temperatures rise, many endotherms take advantage of evaporative cooling in the form of sweating or panting

The advantage of endothermy is that it allows sustained high-energy activity

-The tradeoff is that the high metabolic rate requires constant and high energy intake (food)

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Endotherms

In animal physiology, size does matter!

-Smaller animals have much higher metabolic rates per unit body mass relative to larger animals

-Small endotherms in cold environments require significant insulation to maintain their body temperature

-Large endotherms in hot environments usually have little insulation

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Endotherms

When temperatures fall below a threshold, animals resort to thermogenesis, or use of normal energy metabolism to produce heat

-Shivering thermogenesis uses muscles to generate heat, without producing useful work

-Nonshivering thermogenesis alters fat metabolism to produce heat instead of ATP

-Brown fat is utilized

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Control of Body Temperature

Mammalian thermoregulation is controlled by the hypothalamus

-A rise in body temperature is detected by neurons, which stimulate the heat-losing center in the hypothalamus

-Sympathetic nerves cause dilation of peripheral blood vessels, and

production of sweat from sweat glands

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-A drop in body temperature is detected by neurons, which stimulate the heat-promoting center in the hypothalamus

-Sympathetic nerves cause constriction of peripheral blood vessels, and inhibit sweating to prevent evaporative cooling

-Hypothalamus releases hormones that stimulate the thyroid to produce thyroxin, which stimulates metabolism

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Pyrogens are substances that cause a rise in temperature

-Act on the hypothalamus to increase the normal set point to a higher temperature

-Produce the state we call fever

-A normal response to infection

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Torpor is a state of dormancy produced by a reduction in both metabolic rate and body temperature

-Allows an animal to reduce the need for food intake

Hibernation is an extreme state in which torpor lasts for weeks or months

-Practiced usually by mid-sized animals

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Osmolarity and Osmotic Balance

To maintain osmotic balance, the extracellular compartment of an animal’s body must be able to take water from and excrete excess water into the environment

-Inorganic ions must also be exchanged to maintain homeostasis

-These exchanges occur across specialized epithelial cells, and, in most vertebrates, through the kidney

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Osmotic pressure is the measure of a solution’s tendency to take in water by osmosis

Osmolarity is the number of osmotically active moles of solute per liter of solution

Tonicity is the measure of a solution’s ability to change the volume of a cell by osmosis

-Solutions may be hypertonic, hypotonic, or isotonic

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Osmoconformers are organisms that are in osmotic equilibrium with their environment

-Include most marine invertebrates, and cartilaginous fish (sharks and relatives)

All other vertebrates are osmoregulators

-Maintain a relatively constant blood osmolarity despite different concentrations in their environment

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Freshwater vertebrates are hypertonic to their environment -Have adapted to prevent water from entering their bodies, and to actively transport ions back into their bodies

Marine vertebrates are hypotonic to their environment-Have adapted to retain water by drinking seawater and eliminating the excess ions through kidneys and gills

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Osmoregulatory Organs

In many animals, removal of water or salts is coupled with removal of metabolic wastes through the excretory system

A variety of mechanisms have evolved to accomplish this

-Single-celled protists use contractile vacuoles

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Invertebrates use specialized cells & tubules

-Flatworms use protonephridia which branch into bulblike flame cells

-Open to the outside of the body, but not to the inside

-Earthworms use nephridia

-Open both to the inside and outside of the body

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Insects use Malpighian tubules, which are extensions of the digestive tract

-Waste molecules and K+ are secreted into tubules by active transport

-Create an osmotic gradient that draws water into the tubules by osmosis

-Most of the water and K+ is then reabsorbed into the open

circulatory system through hindgut epithelium

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The kidneys of vertebrates consist of thousands of repeating units, nephrons-Create a tubular fluid by filtering the blood under pressure through the glomerulus-Filtrate contains many small molecules, in addition to water and waste products

-Most of these molecules and water are reabsorbed into the blood

-Waste products are eliminated from the body in the form of urine

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Evolution of the Vertebrate Kidney

Kidneys are thought to have evolved among the freshwater teleosts, or bony fishes

-Body fluids are hypertonic with respect to surrounding water, causing two problems

1. Water enters body from environment

-Fishes do not drink water and excrete large amounts of dilute

urine

2. Solutes tend to leave the body

-Reabsorb ions across nephrons

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In contrast, marine bony fishes have body fluids that are hypotonic to seawater

-Water tends to leave their bodies by osmosis across their gills

-Drink large amounts of seawater

-Actively transport monovalent ions out of the blood across the gill surfaces

-Excrete urine isotonic to body fluids

-Contains divalent cations


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Cartilaginous fish, including sharks and rays, reabsorb urea from the nephron tubules

-Maintain a blood urea concentration that is 100 times higher than that of mammals

-Blood is isotonic to surrounding sea

-These fishes do not need to drink seawater or remove large amounts of ions from their bodies


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The amphibian kidney is identical to that of freshwater fish

The kidneys of reptiles are very diverse-Marine reptiles drink seawater and excrete an isotonic urine

-Eliminate excess salt via salt glands-Terrestrial reptiles reabsorb much of the salt and water in their nephron tubules

-Don’t excrete urine, but empty it into cloaca


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Mammals and birds are the only vertebrates that can produce urine that is hypertonic to body fluids

-Accomplished by the loop of Henle

Birds have relatively few or no nephrons with long loops, and so cannot produce urine as concentrated as that of mammals

-Marine birds excrete excess salt from salt glands near the eyes


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Nitrogenous Wastes

When amino acids and nucleic acids are catabolized, they produce nitrogenous wastes that must be eliminated from the body

-First step is the removal of the amino (-NH2) group, and its combination with H+ to form ammonia (NH3) in the liver

-Toxic to cells, and thus it is only safe in dilute concentrations

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Nitrogenous Wastes

Bony fishes and amphibian tadpoles eliminate most of the ammonia by diffusion via gills

Elasmobranchs, adult amphibians, and mammals convert ammonia into urea, which is soluble in water

Birds, terrestrial reptiles, and insects convert ammonia into the water-insoluble uric acid

-Costs most energy, but saves most water

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Nitrogenous Wastes

Mammals also produce uric acid, but from degradation of purines, not amino acids

-Most have an enzyme called uricase, which convert uric acid into a more soluble derivative called allantoin

-Humans lack this enzyme

-Excessive accumulation of uric acid in joints causes gout

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The Mammalian Kidney

Each kidney receives blood from a renal artery, and produces urine

-Urine drains from each kidney through a ureter into a urinary bladder

Within the kidney, the mouth of the ureter flares open to form the renal pelvis

-Receives urine from the renal tissue

-Divided into an outer renal cortex and inner renal medulla

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The kidney has three basic functions

-Filtration = Fluid in the blood is filtered out of the glomerulus into the tubule system

-Reabsorption = Selective movement of solutes out of the filtrate back into the blood via peritubular capillaries

-Secretion = Movement of substances from the blood into the extracellular fluid, then into the filtrate in the tubular system

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Each kidney is made up of about 1 million functioning nephrons

-Juxtamedullary nephrons = Have long loops that dip deeply into the medulla

-Cortical nephrons = Have shorter loops

Blood is carried by an afferent arteriole to a tuft of capillaries in cortex, the glomerulus

-Blood is filtered as it is forced through porous capillary walls

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Blood components that are not filtered drain into an efferent arteriole, which empties into peritubular capillaries

Glomerular filtrate enters the first region of the nephron tubules, Bowman’s capsule

-Goes into the proximal convoluted tubule

-Then moves down the medulla and back up into cortex in the loop of

Henle

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After leaving the loop, the fluid is delivered to a distal convoluted tubule in the cortex

-Drains into a collecting duct

-Merges with other collecting ducts to empty its contents, now called urine, into the renal pelvis

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Reabsorption and Secretion

Most of the water and dissolved solutes that enter the glomerular filtrate must be returned to the blood by reabsorption-Water is reabsorbed by the proximal convoluted tubule -Reabsorption of glucose and amino acids is driven by active transport carriers

Secretion of waste products involves transport across capillary membranes and kidney tubules into the filtrate

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Excretion

A major function of the kidney is elimination of a variety of potentially harmful substances that animals eat and drink

-In addition, urine contains nitrogenous wastes, and may contain excess K+, H+ and other ions that are removed from blood

Kidneys are critically involved in maintaining homeostasis

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Transport in the Nephron

A mechanism is needed to create an osmotic gradient between the glomerular filtrate and the blood, to allow reabsorption of water

-Virtually all nutrient molecules in the filtrate, and two-thirds of the NaCl and water, are reabsorbed by proximal convoluted tubule

-Active transport of Na+ out of proximal tubule is followed by passive

movement of K+ and water

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The function of the loop of Henle is to create a gradient of increasing osmolarity from the cortex to the medulla -Active extrusion of NaCl from the ascending loop creates an osmotic gradient

-Allows reabsorption of water from descending loop and collecting duct-The two limbs of the loop form a countercurrent multiplier system, that creates a hypertonic renal medulla

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Filtrate that reaches distal convoluted tubule and enters the collecting duct is hypotonic

-The hypertonic interstitial fluid of the renal medulla pulls water out of the collecting duct and into the surrounding blood vessels

Kidneys also regulate electrolyte balance in the blood by reabsorption and secretion

-K+, H+, and HCO3–

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Hormones Control Osmoregulation

Kidneys maintain relatively constant levels of blood volume, pressure, and osmolarity

-Also regulate the plasma K+ and Na+ concentrations and blood pH within narrow limits

-These homeostatic functions of kidneys are coordinated primarily by hormones

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Antidiuretic hormone (ADH) is produced by the hypothalamus and secreted by the posterior pituitary gland

-Stimulated by an increase in the osmolarity of blood

-Causes walls of distal tubule and collecting ducts to become more permeable to water

-Increases reabsorption of water

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Aldosterone is secreted by the adrenal cortex

-Stimulated by low levels of Na+ the blood

-Causes distal tubule and collecting ducts to reabsorb Na+

-Reabsorption of Cl– and water follows

Low levels of Na+ the blood are accompanied by a decrease in blood volume

-Renin-angiotensin-aldosterone system is activated

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Atrial natriuretic hormone opposes the action of aldosterone in promoting salt and water retention

-Secreted by the right atrium of the heart in response to an increased blood volume

-Promotes the excretion of salt and water in the urine and lowering blood volume

temperature, osmotic regulation and the urinary system chapter 49

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